408 E. 0. Hovey — Cherts of Missouri. 



while others contend that the silica of the flints is entirely due 

 to chemical precipitation. Other theories lie between these 

 extremes. A concise summary of the various theories which 

 have been proposed to account for the flints is given by Prof. 

 Prestwich,* who gives as his own opinion the theory that 

 silica in the colloid or soluble state was present as a chemical 

 precipitate in the mud of the Chalk seas and that this colloid 

 silica, having a strong affinity for other forms of silica, gelati- 

 nous substances (like the sarcode of sponges) and other foreign 

 bodies, aggregated about sponge spicules, replacing the sarcode 

 as that decayed, and about the tests of echinoderms and the 

 shells of molluscs. The irregular masses thus produced con- 

 tinued to grow as long as there was any colloid silica within 

 the range of attraction. 



The Corniferous hornstone partakes so largely of the char- 

 acteristics of the Chalk flints that it probably had its origin in 

 the same way.+ 



The Upper Carboniferous cherts of Ireland were studied by 

 Messrs. Hull and Hardman,;}: who came to the conclusion that 

 the chert was "essentially a pseudomorphic rock consisting of 

 gelatinous silica replacing limestone of organic origin chiefly 

 foraminiferal, crinoidal and coralline," the silica being a chem- 

 ical precipitate from the sea water of the period. M. A. 

 Renard§ came to a similar conclusion for the cherts of Carbon- 

 iferous age in Belgium. This view has been vigorously com- 

 batted by Dr. Geo. J. Hinde,| who, starting from the dis- 

 covery made by Prof . Sollas that there were sponge spicules in 

 the Irish cherts, had numerous thin sections of the rocks made 

 for himself. From the study of these Dr. Hinde decided that 

 the cherts were wholly organic in origin and that they were 

 due to the aggregation and disintegration of the skeletons of 

 siliceous sponges. 



Pre-Cambrian cherts have been studied by Irving and Yan 

 Hise^ in their work on the Penokee iron-bearing series of 

 Michigan and Wisconsin. This series belongs to the Huronian 

 subdivision of the Algonkian system and two of its four mem- 

 bers contain much chert. The author's conclusion regarding 

 the origin of the chert is, " First, that the chert was mainly 

 deposited simultaneously with the iron carbonate with which 

 it was so closely associated ; and, second, that it is probable 

 that the chert is of organic origin, although we have no posi- 



*Op. cit., pp. 320-324. Vid. also, Ark. rep't cit., pp. 177-187, where aa 

 extended summary of theories is given. 



f Cf. Manual of Geology, J. D. Dana, 3d edition, 1880, p. 257. 



tSci. Trans. Roy. Dub. Soc, II, vol. 1, pp. 71 and 85, 1878. 



§ Op. cit. 



|| Geol. Mag, N. S., Dec, III, vol. iv, p. 435, 1887. 



\ Tenth Ann. Rep. TJ. S. G. Si, p. 347. 



